The present invention relates to a GPS reception system which receives radio waves emitted from a GPS satellite and carries out positioning.
In recent years positioning systems making use of GPS (Global Positioning System) technology have been widely used in car navigation systems and for geographic surveys. Recently, GPS receivers have been formed integrally with personal computers and in addition to their map navigation function, their use as portable communication tools provided with, a PDA (Personal Data Assist) function, a camera function, a communication function with a linkage to a portable telephone etc., and the like is spreading.
In such a portable GPS system, a GPS receiver which receives radio waves emitted from a GPS satellite, and a processing unit for decoding a received signal and carrying out an arithmetic operation for calculating position information are provided as one integrated structure. An antenna for receiving a radio wave emitted from the GPS satellite is contained in the GPS receiver.
Providing an antenna within the GPS receiver makes it convenient in portability, handling and the like. On the other hand, however, the receiver can not be always located at a place where a sufficient reception level can be provided for positioning. For example, when being used in a vehicle, there have been problems wherein the radio waves emitted from the GPS satellite were obstructed by the body of the vehicle making it difficult to ensure good reception conditions.
The GPS system adopts a spread spectrum method to make the system less susceptible to the influence of multipath fading. However, differences in the direction of arrival of the emitted radio waves for each of GPS satellites cause an additional problem in that positioning accuracy is affected in a strong multipath fading environment.
The present invention was made for solving the above problems with an object of ensuring a higher reception performance. For this, another external antenna is separately provided from the built-in antenna so that it can be connected to the GPS receiver. The built-in antenna is switched to the external antenna when a sufficient reception level for positioning cannot be obtained using only the built-in antenna.
The present invention is provided for achieving the above object. The invention according to one embodiment is a GPS reception system comprising a GPS receiver having a built-in antenna and an external antenna made connectable to the GPS receiver, which system comprises an antenna switching unit which switches between the built-in antenna and the external antenna, and, when the external antenna is connected to the GPS receiver, switching a receiving antenna from the built-in antenna to the external antenna.
According to such a GPS reception system, the external antenna is used when a sufficient reception level cannot be obtained with only the built-in antenna, making it possible to obtain positioning data and to ensure higher reception levels.
The invention according to another embodiment is a GPS reception system which comprises an antenna switching circuit which, when the external antenna is connected to the GPS receiver, detects the connection of the external antenna by detecting the current consumed in the external antenna in order to drive a changeover switch.
According to such a GPS reception system, it becomes possible to switch from the built-in antenna to the external antenna by a simple circuit configuration.
The invention according to yet another embodiment is an antenna switching unit in which the antenna switching circuit comprises a current detecting resistor which is inserted in a power supply line of the external antenna, a comparator which detects a change in a voltage generated across both ends of the current detecting resistor by the consumed current, and a changeover switch driven by an output control signal of the comparator.
According to such a GPS antenna switching unit, it becomes possible to carry out switching from the built-in antenna to the external antenna by a simple and inexpensive circuit configuration.
The invention according to still yet another embodiment is an antenna switching circuit which is configured such that the antenna switching circuit is supplied with two different power supply voltages, of which a first power supply voltage is supplied as a power supply voltage of the comparator, and a second power supply voltage supplies as a voltage, which is set lower than the first power supply voltage, to the power supply line of the external antenna.
According to such an antenna switching unit, it becomes possible to carry out switching from the built-in antenna to the external antenna with a circuit configuration which is inexpensive yet stable in operation without using an expensive comparator such as a rail-to-rail type. Moreover, the second power supply voltage is obtained with the first power supply voltage regulated by a regulator. Thus, it becomes possible to supply a voltage with few ripples and high voltage accuracy.
The invention according to another embodiment is a GPS reception system which has a configuration such that the antenna switching unit comprises a switch which is inserted through a connector for connecting of the external antenna to the GPS receiver, and which switches the reception antenna from the built-in antenna to the external antenna when the external antenna is connected to the GPS receiver.
According to such a GPS reception system, it becomes possible to carry out switching from the built-in antenna to the external antenna without using an antenna switching circuit which uses a switching element, a comparator and the like by simply detecting connection of the external antenna.
The invention according to yet another embodiment is a GPS reception system comprising a GPS receiver having a built-in antenna and an external antenna made connectable to the GPS receiver, which system comprises a reception level detecting unit which measures a reception level of the built-in antenna and that of the external antenna and compares the measured reception levels with each other; and an antenna switching control unit which selects a reception antenna from the measurement and comparison results of the reception levels obtained in the reception level detecting unit, carries out control of switching between the built-in antenna and the external antenna, and carries out diversity reception with the built-in antenna and the external antenna.
According to such a GPS reception system, when a sufficient reception level cannot be obtained using only the built-in antenna, switching is carried out therefrom to the external antenna as appropriate. Therefore, it becomes possible to ensure higher reception performance compared with a GPS reception system with only the built-in antenna.
The invention according to still yet another embodiment is a GPS reception system having a configuration wherein the diversity reception measures reception levels of radio waves emitted from three or more specified GPS satellites for both the built-in antenna and the external antenna, compares the measured reception levels with one another, and selects the antenna from which the highest reception level was obtained as the reception antenna.
According to such a GPS reception system, it becomes possible to obtain positioning data necessary for two dimensional positioning (latitude, longitude) with high reception performance.
The invention according to another embodiment is a GPS reception system which has a configuration wherein the diversity reception periodically carries out switching between the built-in antenna and the external antenna, and selects the antenna from which a higher reception level is obtained as the reception antenna.
According to such a GPS reception system, the antenna with the higher reception level is selected each time. Thus, it becomes possible to always ensure high reception performance.
The invention according to yet another embodiment is a GPS reception system comprising a GPS receiver having a built-in antenna and an external antenna which can be connected to the GPS receiver, which system comprises a reception level detecting unit which measures the reception level of the built-in antenna and that of the external antenna and compares the measured reception levels with each other, and an antenna switching control unit which selects a reception antenna from the measurement and comparison results of the reception levels obtained in the reception level detecting unit and performs control of the switching between the built-in antenna and the external antenna, and, of the built-in antenna or the external antenna, designating the antenna which provides a higher reception level as the reception antenna for each of GPS satellites picked up.
According to such a GPS reception system, the antenna which provides a higher reception level is designated to the of GPS satellites each having arriving radio waves having different directions. Thus, it becomes possible to ensure further higher reception performance.